[X86] Add X86ISD::CMOV to computeKnownBitsForTargetNode and ComputeNumSignBitsForTargetNode.

Summary: Implementations based on ISD::SELECT.

Reviewers: RKSimon, spatel

Reviewed By: RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38663

llvm-svn: 315153